The invention relates generally to heart monitoring systems and more particularly to heart rate monitoring systems, particularly in the presence of artificial heart pacer signal artifacts.
In the art of heart monitoring, particularly to determine a patient's heart rate or the like, thoracic electrical potentials of the patient are sensed to provide an input signal to amplifying means which amplify and process the signal for application to heart rate indicating means and/or display means. In the particular instance of heart rate monitoring, the heart rate indicator is normally responsive only to sensed signals which correspond in frequency and amplitude substantially with the QRS complex or the R-wave of a natural heart beat appearing in the ECG. However, in the event the patient is receiving artificial stimulation as by an implanted heart pacer, the pacer's stimulation pulse may additionally introduce signal amplitudes and/or frequencies to the sensed ECG signal which may be inaccurately identified as a naturally occurring heart rhythm and, in the worse case may provide an indication of continuing heart activity when in fact heart activity has ceased and the patient is technically dead.
To avoid the foregoing problem, the prior art has provided various circuit means for suppressing the pacer discharge pulse portion of the pacer signal artifact, usually be preventing transmission of the sensed ECG signal to the heart rate indicating circuitry for the duration of the pacer discharge pulse.
Heart pacers are normally operated such that there is no resultant or net polarization of their electrodes, and thus may be said to operate in an AC mode. The most effective way of accomplishing this AC mode of pacer operation is through capacitively coupling the heart stimulating energy to the heart. However because of such capacitive coupling and AC operation, the main discharge of the energy-storing capacitor to provide the stimulation pulse is followed by a recharge of the capacitor. The recharging capacitor results initially in the sensed ECG exhibiting an overshoot voltage having the opposite polarity to the heart stimulation pulse, which overshoot decays exponentially to a zero voltage value at a rate determined by the time constant of the capacitor recharge path. The resulting recharge voltage or waveform, sometimes known as a pacer "tail," has recently been recognized as having ample amplitude and a frequency spectrum in the domain of the QRS complex such as to be able to trigger or enter a false count on the heart rate monitor.
Accordingly, it is a principle object of the present invention to provide an improved heart monitoring system having heart rate indicating means. Included in this object is the provision of means for preventing false actuation of the heart rate indicating means by any portion of a heart pacer signal artifact.